This invention relates to roller tappets of the type including a central valve lifter body having a bifurcated end for support of a roller. The roller tappet is adapted for placement within a cast iron roller tappet bore defined by an engine block and the roller is engaged by cam lobes formed on a cam shaft.
Roller tappets have been employed to increase engine breathing since they allow increased lift velocity without an increase in the tappet body diameter. Conventionally, roller tappets include a central valve lifter body portion having a bifurcated end in which the roller is mounted. The roller tappet body reciprocates in a bore formed in the engine block. The roller tappet bore is dimensioned to provide sufficient guide surface area for proper operation of the tappet.
Conventional, non-roller hydraulic tappets are normally fabricated from hardenable or chilled gray iron. These materials are not strong enough for use with roller tappets. The legs of the bifurcated end which supports the roller when formed from these materials will not withstand loads imposed on them during operation. As a result, the industry has universally specified SAE 1144 steel having a tensile strength of approximately 130,000 PSI in the fabrication of roller tappets. SAE 1144 steel has been felt to be the material having sufficient strength and wearability to provide the roller tappets with an adequate service life.
Although these prior roller tappet constructions having a valve lifter body fabricated from steel have generally been acceptable, several problems have been encountered. For example, when used in an engine block formed from cast iron, side wall and guide surface wear have been experienced. Further, the steel tappet bodies have a tendency to gall or chafe within the cast iron roller tappet bores. This wear and galling increase the tolerances and result in improper tappet operation.